The FL being a guide has to watch ML have s*x with other women for purification purpose. Though described as solely a mechanical act, it still left a lingering uneasy feeling, thinking how both of them are gonna end up together. It was a little unbelievable in the start but became normal as I read it just as the basic plotline of the story. If you get past this, it gets better. Also she has to watch it like 5 or 6 times before ML starts feeling uneasy himself. His realization is what makes all this worth it.

Like in many guideverse stories s*x is a necessity for ability users (magic here). Those with magic and holy power (purification/guiding) have pheromones like in omegaverse. Poor ML suffers from sensitivity to most pheromones and they smell like rotting corpses to him. This prevents him from getting pheromone purification. He has to force himself to have s*x with those who have high Holy power which is physically repulsive to him due to his pheromone sensitivity.

In this world almost everyone has magic power and many also have holy power. It's very rare to not have magic power and even rarer to just have holy power. Since ML can't stand the pheromones of those with magic power he finds the MC a magic-less person, who happens to be a rare case with Holy power. This allows her to act as a guide for the ML but since her Holy power level is low he hires her to help relieve the repulsion of his sexual/guiding partners pheromones and that's why she sits in the room behind a curtain while he has s*x. MC treats it like providing a medical service and even finds some interest in it.

His madness is heavily influenced by his lack of purification and as he improves with the MC's help he gets increasingly stable and sweet. A lot of his poor EQ is also due to a grim setting of his family so he's very lacking in emotional development but improves quickly with the MC's guidance.

how she'll be present when he's having s*x